1. Field of the Invention
This invention relates to a steering assembly for use in a bicycle, tricycle or similar vehicle. The invention has particular application to steering assemblies incorporating threadless steerer tubes.
2. Description of the Related Art
Prior art bicycle steering assemblies commonly include a front wheel mounted in a fork assembly. The fork assembly has a steerer tube which extends upwardly through a head tube mounted to the bicycle frame. The upper end of the steerer tube projects past the top of the head tube. The steerer tube is pivotally mounted to the head tube by bearing assemblies at either end of the head tube. During normal use the bearing assemblies may be exposed to very large forces. The bearing assemblies typically each comprise upper and lower races separated by a plurality of ball bearings which roll on bearing surfaces of the upper and lower races.
Applying the correct amount of pre-load force to the bearing assemblies is important to the proper functioning of such prior art steering assemblies. If there is too little pre-load then the bearings will be loose and incapable of transmitting forces from the steerer tube to the head tube without suffering damage. If the bearings are very loose then steering control may be adversely affected. Too much pre-load force can damage the bearings, cause the bearings to bind and make the steering action rough.
In so-called "threaded" prior art steering assemblies the steerer tube is secured by a nut threaded on an externally threaded section of the steerer tube. The lower surface of the nut bears on the upper race of the upper bearing assembly. The lower race of the lower bearing assembly is mounted to the steerer tube. In these steering assemblies, bearing pre-load is adjusted by altering the tightness of the nut. A separate lock-nut is generally provided to lock the nut in position after the bearing pre-load has been set. Such assemblies have the disadvantage that it is difficult to properly set bearing pre-load because tightening the lock-nut generally turns the nut sufficiently to alter the bearing pre-load. With this design it is also difficult to prevent the nut from coming loose during use. Furthermore, providing external threads on the steerer tube is an expensive manufacturing step.
U.S. Pat. No. 5,095,770, Rader III shows a bicycle steering assembly in which the steerer tube is "threadless", that is, it has a smooth outer surface with no external threads. The top end of the steerer tube is threaded internally. An adjustment screw is threaded into the top end of the steerer tube. The adjustment screw applies pressure to the top race of the upper bearing assembly via a mounting sleeve and a tapered compression ring. In a bicycle, handlebars are mounted at one end of a stem attached to the mounting sleeve. Steering assemblies according to the Rader III design have recently become very popular, especially in high quality all terrain bicycles.
In the Rader III assembly, the mounting sleeve is clamped externally to the steerer tube. The mounting sleeve must be fixed relative to the steerer tube because the bicycle handlebar stem is mounted to the mounting sleeve. It is not possible to affix the mounting sleeve by clamping it inside the bore of the steerer tube because of the adjustment screw which threads into the upper end of the steerer tube.
The need to clamp the sleeve externally to the steerer tube causes the Rader III design to have some disadvantages. Fixing the sleeve to the steerer tube generally requires either protruding clamping bolts, which can injure a rider in a fall, or alternative clamping arrangements which are either structurally weak or can cause damage to the steerer tube if over-tightened.
A further disadvantage of the Rader III design is that it is somewhat difficult to properly set the bearing pre-load. This is because there is friction between the sleeve and the steerer tube. To set bearing pre-load, the sleeve must be loosened and slid upward and then the adjustment screw must be tightened just enough to provide the correct bearing pre-load. If the adjustment screw is over-tightened then the adjustment screw must be loosened, the sleeve must be slid upward and the process repeated. Bearing pre-load cannot be reliably reduced by simply loosening the adjustment nut because the sleeve might not slide freely up the steerer tube when the adjustment nut is loosened.
U.S. Pat. No. 5,303,611 Chi discloses an upper steering assembly for a bicycle in which an externally threaded member is clamped to the outside surface of a steerer tube. A locking nut is threaded on the externally threaded member. Bearing pre-load can be set by turning the locking nut relative to the externally threaded member. This design has several parts which are expensive to machine.
U.S. Pat. No. 5,201,242, Chi and U.S. Pat. No. 5,319,993 disclose designs for a bicycle steering assemblies which are similar to the Rader III design but do not require internal threads on the bicycle steerer tube. Like the Rader III design, these designs use stem assemblies which clamp to the outside of the steerer tube. These designs suffer from disadvantages similar to those of the Rader III design.
U.S. Pat. No. 5,303,611, Chi discloses a mechanism for adjusting pre-load on steering bearings in a bicycle equipped with a threadless steerer tube. Chi provides a compression socket which clamps to the steerer tube. The compression socket has external threads around its lower periphery. A rotational socket is threaded to the compression socket. Pre-load on a bearing assembly mounted beneath the rotational socket may be adjusted by turning the rotational socket with respect to the compression socket. The Chi design includes several relatively complicated parts and requires handlebars to be mounted to the steerer tube with separate components.